Hariri was fatally injured and nine other people were killed whan an apparent car bomb exploded as his motorcade drove along a beachside boulevard. The blast injured about a hundred people and severely damaged nearby buildings.
Nobody has claimed responsibility for the assassination.
In Damascus, Syrian leaders, including President Bashar al-Assad, condemned the killing. Foreign Minister Farouk al-Sharaa said: "We condemn those who are sowing seeds of conflict in Lebanon, our brotherly country. We hope that the Lebanese people will be solid and strong in these difficult moments, that they reject any internal conflict and any external interference. There should be an investigation by Lebanese authorities to see who was behind this act."
In an initial reaction, White House spokesman Scott McClellan denounced the killing, saying it was a reminder that the Lebanese people must be able to build a future "free from Syrian occupation."
French President Jacques Chirac called for an international inquiry into the bombing.
Hariri, a 60-year-old Sunni Muslim, was the man credited with reviving his country's fortunes after the devastating civil war that ended in 1989. He served as prime minister for 10 years in two spells, resigning in October last year.
He stepped down amid growing divisions in the Lebanese government about the all-pervading influence of Syria in Lebanese politics. Syria has some 20,000 troops in Lebanon, and since the end of the civil war has gradually come to dominate the political scene. Hariri recently joined calls for Syria to remove its troops in Lebanon.
Tributes have been flowing in. Amr Musa, secretary-general of the Arab League, called Hariri one of the most prominent leaders of the Arab world. "I am so sad that we lost Rafiq Hariri, a great Arab leader and a Lebanese figure of a very respectable stature, who spent his life serving Lebanon, serving the Arab world and the Arab common causes and the common Arab interests," Musa said. "It's a heinous crime."
Michael Glackin, the managing editor of the Beirut newspaper "The Daily Star," said Hariri will be sorely missed. "Certainly, it has created a massive vacuum in Lebanese politics," Glackin said. "He was internationally well-known, internationally respected as somebody who was a billionaire and as a politician."
EU High Representative for Common Foreign and Security PolicyJavier Solana described him as a man of peace who managed to rebuild his homeland after years of war.
Some speculated the attack may also have been aimed Hariri's business interests. He was a multimillionaire
real-estate developer who also had a stake in the computer industry.For more on the regional implications of events in Lebanon, see "Implications Of New International Consensus On Lebanon" in "RFE/RL Iran Report".